.\" Copyright (C) 2022 Jens Axboe <axboe@kernel.dk>
.\"
.\" SPDX-License-Identifier: LGPL-2.0-or-later
.\"
.TH io_uring_prep_recvmsg 3 "March 12, 2022" "liburing-2.2" "liburing Manual"
.SH NAME
io_uring_prep_recvmsg \- prepare a recvmsg request
.SH SYNOPSIS
.nf
.B #include <sys/types.h>
.B #include <sys/socket.h>
.B #include <liburing.h>
.PP
.BI "void io_uring_prep_recvmsg(struct io_uring_sqe *" sqe ","
.BI "                           int " fd ","
.BI "                           struct msghdr *" msg ","
.BI "                           unsigned " flags ");"
.PP
.BI "void io_uring_prep_recvmsg_multishot(struct io_uring_sqe *" sqe ","
.BI "                                     int " fd ","
.BI "                                     struct msghdr *" msg ","
.BI "                                     unsigned " flags ");"
.fi
.SH DESCRIPTION
.PP
The
.BR io_uring_prep_recvmsg (3)
function prepares a recvmsg request. The submission queue entry
.I sqe
is setup to use the file descriptor
.I fd
to start receiving the data indicated by
.I msg
with the
.BR recvmsg (2)
defined flags in the
.I flags
argument.

This function prepares an async
.BR recvmsg (2)
request. See that man page for details on the arguments specified to this
prep helper.

The multishot version allows the application to issue a single receive request,
which repeatedly posts a CQE when data is available. It requires the
.B IOSQE_BUFFER_SELECT
flag to be set and no
.B MSG_WAITALL
flag to be set.
Therefore each CQE will take a buffer out of a provided buffer pool for receiving.
The application should check the flags of each CQE, regardless of its result.
If a posted CQE does not have the
.B IORING_CQE_F_MORE
flag set then the multishot receive will be done and the application should issue a
new request.

Unlike
.BR recvmsg (2),
multishot recvmsg will prepend a
.I struct io_uring_recvmsg_out
which describes the layout of the rest of the buffer in combination with the initial
.I struct msghdr
submitted with the request. See
.BR io_uring_recvmsg_out (3)
for more information on accessing the data.

Multishot variants are available since kernel 6.0.

After calling this function, additional io_uring internal modifier flags
may be set in the SQE
.I ioprio
field. The following flags are supported:
.TP
.B IORING_RECVSEND_POLL_FIRST
If set, io_uring will assume the socket is currently empty and attempting to
receive data will be unsuccessful. For this case, io_uring will arm internal
poll and trigger a receive of the data when the socket has data to be read.
This initial receive attempt can be wasteful for the case where the socket
is expected to be empty, setting this flag will bypass the initial receive
attempt and go straight to arming poll. If poll does indicate that data is
ready to be received, the operation will proceed.

Can be used with the CQE
.B IORING_CQE_F_SOCK_NONEMPTY
flag, which io_uring will set on CQEs after a
.BR recv (2)
or
.BR recvmsg (2)
operation. If set, the socket still had data to be read after the operation
completed. Both these flags are available since 5.19.
.P

.SH RETURN VALUE
None
.SH ERRORS
The CQE
.I res
field will contain the result of the operation. See the related man page for
details on possible values. Note that where synchronous system calls will return
.B -1
on failure and set
.I errno
to the actual error value, io_uring never uses
.IR errno .
Instead it returns the negated
.I errno
directly in the CQE
.I res
field.
.SH NOTES
As with any request that passes in data in a struct, that data must remain
valid until the request has been successfully submitted. It need not remain
valid until completion. Once a request has been submitted, the in-kernel
state is stable. Very early kernels (5.4 and earlier) required state to be
stable until the completion occurred. Applications can test for this
behavior by inspecting the
.B IORING_FEAT_SUBMIT_STABLE
flag passed back from
.BR io_uring_queue_init_params (3).
.SH SEE ALSO
.BR io_uring_get_sqe (3),
.BR io_uring_submit (3),
.BR recvmsg (2)
